The present invention relates generally to a production method of a phase shift photomask used to form fine patterns at high density, which are used for the production of high-density integrated circuits such as very LSIs and ultra-very LSIs, and more particularly to a production method of a phase shift photomask having a phase shift layer comprising spin-on glass (SOG for short).
As the degree of integration of semiconductor integrated circuits becomes higher, reticles used for circuit fabrication are now required to become finer and finer. At present, line widths of device patterns transferred from five-fold reticles for DRAMs of 16 megabytes are as fine as 0.6 .mu.m. Device patterns for DRAMS of 64 megabytes are now required to have a resolution of 0.35 .mu.m line width, and so can no longer be achieved by conventional light exposure systems using steppers.
To achieve this by forming fine patterns, methods of forming such fine patterns by making wavelengths of exposure light sources short, making transfer lenses have a high NA, and using zonal illumination, or methods of forming patterns by direct electron beam lithography without recourse to any photomask are now under investigation. A grave problem with such pattern-forming methods are, however, that some added cost is needed because of renovation of existing exposure systems, and introduction of new equipment.
For this reason, attention is now directed to pattern transfer methods using a phase shift photomask, which enable currently available steppers to be used for the formation of fine patterns.
Basic concepts and principles of a phase shift photomask have already been disclosed in JP-A-58-17344 and JP-B-62-59296. A particular merit of the phase shift photomask, which makes it possible to use existing exposure systems without giving any modification thereto, is now taken into reconsideration, and so various types of phase shift photomasks are under active development.
A brief account will now be given of the principles of transfer using a phase shift photomask with reference to FIG. 5. For the purpose of comparison, a transfer method using a conventional photomask will be explained with reference to FIG. 6, thereby explaining a resolution difference between both methods.
FIG. 5(a) is a diagram illustrating how to use a phase shift photomask 500 to conduct projection alignment using exposure light 550, and FIGS. 5(b) and 5(c) show a light amplitude profile and a light intensity profile, respectively, as measured on a resist-on-wafer. FIG. 6(a) is a diagram illustrating how to use a conventional photomask 600 to conduct projection alignment using exposure light 650, and FIGS. 6(b) and 6(c) show a light amplitude profile and a light intensity profile, respectively, as measured on a resist-on-wafer.
In FIGS. 5(a) and 6(a), reference numerals 510 and 610 stand for a transparent substrate, 520 an etching stopper layer, 530 and 630 a block (chromium) film, 540 a shifter, 550 and 650 exposure light (ionizing radiation), 500 a phase shift photomask, and 600 a conventional photomask.
The phase shift photomask 500 shown in FIG. 5(a) is made up of the transparent substrate 510, a line-and-space pattern formed of the block films 530 at a given width and pitch and located on the transparent substrate 510, openings provided at every one such line-and-space pattern, and shifter layers 540, each located astride the block layers 530 adjoining one opening, and the conventional photomask 600 shown in FIG. 6(a) is built up of the transparent substrate 610, and a line-and-space pattern composed of the block films 630 at a given width and pitch and located on the transparent substrate 610. It is here to be noted that the etching stopper layer 520 is provided all over the surface of the transparent substrate 510 between the substrate 510 and each block layer 530.
As the exposure light 550 is incident on the phase shift photomask 500, the amplitude of the light transmitting through the shifter portion 540 is shifted from the amplitude of the light transmitting between the shifter-free block films 530 by a phase n .pi. (n is an odd number), and inverted on the emerging side. For this reason, these lights interfere with each other on the resist-on-wafer to have such an amplitude profile as shown in FIG. 5(b) and, hence, such a light intensity profile as shown in FIG. 5(c).
In the case of the arrangement using the conventional photomask 600, however, the amplitude of the light emerging from the photomask has such a profile on the resist-on-wafer as shown in FIG. 6(b), because the light emerging from the openings is in phase and so interferes with each other. Consequently, the emerging light has such an intensity profile on the resist-on-wafer as shown in FIG. 6(c).
In the light intensity profile shown in FIG. 5(c) there are points between knolls, at which the intensity of the light becomes zero, while in the profile shown in FIG. 6(c) the profile form of light intensity spreads out. In other words, the light intensity profile shown in FIG. 5(c) is superior to that shown in FIG. 6(c) in terms of resolution on the resist-on-wafer.
Thus, the transfer method using the phase shift photomask 500 is found to be superior to that using the conventional photomask 600 in terms of resolution, so that the former can achieve transfer of finer patterns than would be possible with the latter.
The phase shift photomask shown in FIG. 5 is referred to as a Levenson type of phase shift photomask. Many other phase shift photomasks varying largely in structure are available, for instance, halftone and auxiliary types, and they work on the same basic concepts and principles and so may be used depending on what purpose they are used for. The Levenson type in particular is alleged to be effective to improve the resolution of a line-and-space or other pattern.
For the Levenson type of phase shift photomask shown in FIG. 5, SOG (spin-on glass) that is a coated type of silicon oxide is commonly used.
The SOG has been used not only for the aforesaid Levenson type of phase shift photomask but also for an edge block type of phase shift photomask such as one shown in FIG. 7(c) and a rim type of phase shift photomask such as one shown in FIG. 7(d).
It is here to be noted that the phase shift photomask shown in FIG. 5 is of the same type as shown in FIG. 7(a), wherein a shifter layer is laid over a block layer of a Levenson type of phase shift photomask, and so is called a shifter overlaid type of Levenson phase shift photomask. What is shown in FIG. 7(b) is, on the other hand, called a shifter underlay type of Levenson phase shift photomask wherein, as illustrated, a shifter layer 730B is situated under a block layer 740B.
The thicknesses of the shifter layers 730A, 730B, 730C and 730D, each composed of SOG, are controlled such that the phase of exposure light used for transfer is shifted by n .pi. (n is an odd number) and inverted when it passes at its wavelength through them.
It is to be understood that in FIGS. 7(a) to 7(d) transparent substrates are represented by 710A, 710B, 710C and 710D, etching stopper layers by 720A, 720B, 720C and 720D, and block (chromium) films by 740A and 740D.
As mentioned above, the SOG has commonly been used for shifter layers, and patterned as mentioned below.
This will now be explained typically with reference to the production of a shifter overlaid type of phase shift photomask.
FIG. 3 illustrates a first production method wherein an SOG coated on a substrate is selectively irradiated with electron radiation or laser light (e.g., Ar laser light of 363.8 nm wavelength), and then developed with a solvent to leave the portion irradiated with laser light while removing the unexposed portion, thereby patterning the SOG. First, provision is made of a blank 310 in which block layers 312 formed of chromium and arranged according to a given pattern are formed on a transparent substrate 311 (see FIG. 3(a)).
Following this, an SOG 340 was coated all over the surface of the substrate on which the block films 312 are formed, and then subjected to soft baking at 80.degree. to 120.degree. C. (see FIG. 3(b)).
Then, only given regions of the SOG are selectively irradiated with electron radiation 360 (see FIG. 3(c)).
Subsequently, the unexposed portion is removed by a development treatment while the portion irradiated with electron radiation or laser light 360 is kept intact, thereby obtaining a desired form of SOG shifter pattern 330A (see FIG. 3(d)).
After this, the SOG shifter pattern 330A is fired (see FIG. 3(e)), thereby obtaining a phase shift photomask 300 having the desired form of SOG shifter pattern 330A.
Firing is done by heating at 400.degree. to 500.degree. C. for the purpose of making an intimate SOG film having a high molecular weight.
A brief account will here be given of the SOG itself, and the principles of patterning the SOG.
By the "SOG" used herein is meant a film obtained by the conversion of an organic solvent solution of an organosilicon compound into silicon oxide by coating, drying, and heating. For the starting material for the SOG use may be made of metal alkoxides such as tetraethoxysilane, bipolar solvents such as water and methanol, hydrochloric acid, and the like. To allow methyl groups to remain in the SOG, triethoxymethylsilane, diethoxydimethylsilane, or trimethylethoxysilane is added to the tetraethoxysilane at an amount of a few % to a few tens %. By mixing these starting materials together, hydrolysis and polycondensation occur, resulting in the yielding of an Si--O polymer (polysilicate) having a low molecular weight.
Upon this low-molecular-weight SOG spin-coated on a substrate and subjected to soft baking, there is a slight increase in the molecular weight of the SOG.
Following this, the SOG is selectively irradiated with electron beams, ion beams, radiation light such as X-rays, .gamma.-rays and SOR, or laser light (hereinafter referred to as ionizing radiation). This then causes the irradiated regions to be polymerized resulting in a molecular weight increase. After the irradiation of the SOG with ionizing radiation, the SOG is developed with an alcohol or other solvent, so that the portion irradiated with ionizing radiation is left intact while the unexposed portion is removed due to a molecular weight difference between both the portions, thereby patterning the SOG.
This method has an advantage in that the process involved is simple, but is impractical because the SOG itself has a low sensitivity to ionizing radiation.
Accordingly, the patterning of the SOG has generally been carried out by such a second method as shown in FIG. 4.
This method makes use of a photosensitive resist having a high sensitivity to ionizing radiation (electron radiation) for the purpose of patterning the SOG. The resist coated on the SOG is selectively irradiated with (or exposed to) electron radiation. Thereafter, the resist is developed with a suitable solvent to form a resist pattern through a difference in solubility in the solvent between the exposed and unexposed portions. Then, the SOG is etched in a form conforming to this pattern, thereby forming an SOG shifter pattern.
First, provision is made of a blank 410 in which block films 412 formed of chromium are provided according to a given pattern on a transparent substrate 411 (see FIG. 4(a)), after which an etching stopper layer 420 to be used for the etching of the SOG is formed (see FIG. 4(b)).
Then, an SOG 430 is coated all over the surface of the substrate on which the block films 412 are formed (see FIG. 4(c)). After removal of a peripheral SOG film from the substrate, the SOG 430 is fired (see FIG. 4(d)).
This firing step by heating at 400.degree. to 500.degree. C. yields an intimate SOG film having a high molecular weight.
Thereafter, a photosensitive resist 440 is coated on the SOG 430, and dried (see FIG. 4(e)), after which a conductive layer 450 is formed on the resist for surface conductivity (see FIG. 4(f)). Then, given regions of the conductive layer are selectively irradiated with electron radiation 460 (see FIG. 4(g)).
Subsequently, a resist pattern 440A is formed by a development treatment and drying (see FIG. 4(h)), and the SOG 430 is etched using this resist pattern 440A as an etching-resistant mask (see FIG. 4(i)).
Subsequent removal of the resist pattern 440A yields a phase shift photomask 400 having a desired SOG shifter pattern 430A formed thereon (see FIG. 4(j)).
Thus, the second method is a complicated process involving a number of steps, and so likely to induce many defects.
As described above, the problem with the first method is that it is impractical because the SOG itself has a low sensitivity to ionizing radiation, while the problem with the second method is that it is a complicated process likely to induce many defects. A solution to these problems is now sought out.
Apart from this, it has recently been proposed to increase the sensitivity of a resist by incorporating an acid generator therein. Thus, the aforesaid first method has been modified, too, such that an SOG shifter pattern is fabricated using an acid generator-containing SOG.
However, this method does also offer a problem in that such an acid generator-containing SOG has a low transmittance with respect to light lying in the ultraviolet region.
The leading reason is that the acid generator generally contains an ultraviolet-sensitive group, which absorbs ultraviolet radiation.